In projection displays, for example as used in lecture theatres, a diffuse light source (e.g. a high powered lamp) is used to illuminate an electrically addressed spatial light modulator (EASLM). Projection optics are provided to focus an image of the EASLM to a given image plane, such as a projection screen or a wall. Images formed by one or more EASLMs can be successively transferred to different sub-image regions of an Optically Addressed Spatial Light Modulator (OASLM) using an Active Tiling™ system. Active tiling systems are described further in U.S. Pat. No. 6,437,919 and U.S. Pat. No. 6,654,156, the specifications of which are herein incorporated by reference.
An active tiling system may include one or more EASLMs that are illuminated with diffuse light. A number of images of each EASLM are formed using replication optics and focused to a single active tiling image plane. An OASLM is placed in the active tiling image plane and sub-regions thereof are selectively sensitized, for example by employing patterned OASLM electrodes or by using an array of shutters to sequentially block certain replicated images. The active tiling technique thus permits sub-regions of the OASLM to be successively optically addressed, thereby allowing an image with a very high pixel count to be constructed. This system is may be used for volumetric imaging, as it enables an OASLM to be written with a highly complex binarized Fourier Transform (FT) of a final desired volumetric image. The image can be displayed to a viewer via an appropriate FT lens or mirror system.
Devices of the type described above require that the image or images of each EASLM are focused at a desired focal plane. Accordingly, the OASLM is located accurately in the active tiling image plane to within the depth of focus of the active tiling optics. For example, if the active tiling system is configured to write 6.6 μm pixels the resulting depth of focus is only around 201 μm. If the desired OASLM pixel size is decreased, the depth of focus would also reduce accordingly.
In systems configured to display very large volumetric images, many active tiling channels are used in parallel to produce massive pixel counts in a single active tiling image plane. A single OASLM, or multiple OASLMs, will occupy this image plane but the entire area of the OASLM or OASLMs must lie within the depth of focus (e.g. 20 μm) of the active tiling channels. It follows that the active tiling channels must all be mutually aligned to produce a common focal plane, and it can be seen that such a requirement will place tight constraints on the design and build (and hence overall cost) of the system.